This application claims priority under 35 U.S.C. xc2xa7 xc2xa7 119 and/or 365 to Japan Patent Application No.11-186769 and 11-211519 filed in Japan on Jun. 30, 1999 and Jul. 27, 1999, respectively, the entire content of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a heat resistant Ni base alloy having high strength at high temperature and excellent in hot workability, weldability, and carburization resistance. The alloy of the present invention is suitable in particular as a material of tubes used in naphtha reforming furnaces and ethylene cracking furnaces for producing petrochemical fundamental products such as ethylene and propylene by cracking with steam hydrocarbon materials such as naphtha, propane, ethane, and gas oil at a high temperature of 800xc2x0 C. or more.
2. Description of the Related Art
The service temperature of the tubes used in ethylene cracking furnaces tends to be higher from the viewpoint of improving an ethylene yield ratio. Materials of such cracking furnace tubes require high-temperature strength, carburization resistance and heat resistance because their inside is exposed to a carburization atmosphere.
On the other hand, carbon is deposited on the inside surface of a cracking furnace tube during the operation (this phenomenon is called coking), and as the amount of carbon deposited on the inside of the tube increases, disadvantages in operation occur, such as an increase in the internal pressure of the tube and a reduction in the heating efficiency. Therefor, coking resistance is required. In the actual operation, the so-called decoking operation for removing deposited carbon by air or steam is periodically conducted, but the suspension of the operation and the working load are great problems.
Such coking and its related problems are more serious when the cracking furnace tube has a smaller diameter which is advantageous for improving the yield.
JP Publication No. A 2-3336 discloses the technique of inhibiting coking in which more than 28% Cr is contained in an alloy to form a strong and stable Cr2O3 layer on the surface of the alloy in order to prevent the coking-promoting catalytic elements of Fe and Ni from being exposed to the surface of the alloy.
On the other hand, increasing of the Si content in an alloy is known to be effective for improving carburization resistance, as disclosed in e.g. JP Publication No. A 57-23050.
In the prior art described above, however, there are problems as follows:
When the high-Cr alloy disclosed in JP Publication No. A 2-8336 is applied as a structural member with high-temperature strength for the prevention of coking, the metal structure should be austenitized by increasing the Ni content in the alloy, but, as the result, its high-temperature strength becomes lower than that of the conventional alloy. Therefore, the application thereof as a structural member with high-temperature strength is difficult.
JP Publication No. A 2-336 discloses that an alloy poor in high-temperature strength is combined for use with another member with high-temperature strength to form a cladded tube, but the cladded tube is problematic in respect to the production cost and reliability.
The present inventors found previously that the carburization resistance and coking resistance can be significantly improved by forming a strong and tight Al2O3 layer on the surface of a metal by increasing the content of Al in an alloy, compared with the conventional alloy, and the gxe2x80x2 phase is finely precipitated in the matrix during the service at high temperature by increasing the content of Ni in such a high-Al alloy, and the creep rupture strength can also be significantly improved. The patent for this alloy was applied as a Ni base alloy suitable as a tube in an ethylene cracking furnace in Japanese Patent Application No. 3-308709 (Publication No. A4-358037) and Japanese Patent Application No. 4-41402 (Publication No. A5-2395 77) respectively. However, in consideration of mass production on a commercial scale, an large amount of hot working was required for the production of the Ni base alloy with high-Al but the hot workability of such alloy was not satisfactory.
With respect to the Ni base alloy with an increased content of Al, alloys excellent in oxidation resistance are disclosed in JP Publication No. B 3-46535 and A 60-238434. However, the alloys disclosed in these publications are also poor in hot workability and weldability, because adequate attention was not paid to these characteristics on the design of alloying components. Further alloys excellent in carburization resistance and high-temperature strength are also disclosed in JP Publication No. A 7-54087 and A 9-243284, but attention was not actually paid to hot workability and weldability.
The object of the present invention is to provide a heat resistant alloy which is excellent in carburization resistance and coking resistance under the environment where ethylene cracking furnace tubes are used, more specifically, carburization, oxidation and temperature change are repeated; and also which is excellent in hot workability and weldability and has excellent high-temperature strength.
The summary of the present invention is as follows:
(1) A heat resistant Ni base alloy excellent in hot workability, weldability, and carburization resistance comprising, on a mass% basis, C: 0.1% or less, Si: 2% or less, Mn: 2% or less, S: 0.005% or less, Cr: 10 to 25%, Al: 2.1 to less than 4.5%, N: 0.08% or less, 0.001 to 1% in total of one or more elements of B: 0.03% or less, Zr: 0.2% or less and Hf: 0.8% or less, 2.5 to 15% in total of either one or both of Mo:0.01 to 15% and W: 0.01 to 9%, Ti: 0 to 3%, Mg: 0 to 0.01%, Ca: 0 to 0.01%, Fe: 0 to 10%, Nb: 0 to 1%, V: 0 to 1%, Ta: 0 to 2%, Y: 0 to 0.1%, La: 0 to 0.1%, Ce: 0 to 0.1%, Nd: 0 to 0.1%, Cu: 0 to 5%, Co: 0 to 10%, and the balance being substantially Ni.
(2) The heat resistant Ni base alloy according to above (1), wherein the Ti content is 0.005 to 3% by mass.
(3) The heat resistant Ni base alloy according to above (1), wherein the Ti content is 0.005 to 3% and either one or both of Mg and Ca are contained in an amount of 0.0005 to 0.01% and 0.0005 to 0.01%, respectively.
(4) The heat resistant Ni base alloy according to above (1), wherein the Ti content is 0.005 to 3%, either one or both of Mg and Ca are contained in an amount of 0.0005 to 0.01% and 0.0005 to 0.01%, respectively, and Fe content is 0.1 to 10%.
(5) A heat resistant Ni base alloy comprising, on a mass % basis, C: 0.07% or less, Si: 0.01 to 1%, Mn: 1% or less, S: 0.0025% or less, Cr: 12 to 19%, Al: 2.1 to less than 3.8%, N: 0.045% or less, 0.001 to 1% in total of one or more elements of B: 0.03% or less, Zr: 0.2% or less and Hf: 0.8% or less, Mo: 2.5 to 12%, Ti: 0.005 to 1%, Ca: 0.0005 to 0.01%, Fe: 0.1 to 10%, and the balance being essentially Ni.
In order to obtain the alloy which satisfies the essential characteristics required as a commercial production alloy such as hot workability and weldability without lowering carburization resistance and coking resistance at high temperatures, the present inventors carried out extensive experiments on alloy, with various chemical compositions and reached the following findings as a result.
a) Even if the Ni base alloy does not contain Al in an amount of 4.5% or more unlike the conventional alloy, an alumina-based oxide layer can be formed on the surface of the alloy contenting less than 4.5% Al by means of containing not less than 10% Cr and reducing the N content, whereby excellent carburization resistance and coking resistance can be attained and also high-temperature strength is improved.
b) The N in an alloy containing not less than 1% Al easily forms Al nitride, and the alumina-based oxide layer tends to lose its protective property from this nitride precipitation which acts as the initiation point.
c) By reducing the Al content to less than 4.5%, hot workability and weldability of the Ni base alloy containing Al are improved, but compared with conventional Fexe2x80x94Crxe2x80x94Ni alloys or Nixe2x80x94Cr alloys, are still unsatisfactory in consideration of mass production. This is because Nixe2x80x94Al intermetallic compounds precipitate at the time of hot working or welding, and the inside of the grains of the alloy are significantly reinforced so that the grain boundaries are relatively weakened, which result in both a reduction of hot workability and an occurrence of a hot cracking at the time of welding. Accordingly, the reinforcement of the grain boundaries which can compete with the reinforcement of the inside of the grains is important and effective for improving hot workability and weldability.
d) On the other hand, in the Ni base alloy containing a large amount of Al, the grain boundaries are weakened and S is one of the main factors in weakening the grain boundaries. Therefore, it is important for improving hot workability to limit the S content to 0.005% or less, and further improving effects can be expected by limiting S content to 0.003% or less.
e) Further, the elements, of B, Zr and Hf enhance the binding force of grains on the grain boundaries and are effective for reinforcing the grain boundaries. Therefore, it is preferable that one or more of these elements are contained while the S content is reduced.
f) The reduction of S content as described above and the incorporation of one or more elements of B, Zr and Hf are effective for preventing the reduction of hot workability and the occurrence of hot cracking at the time of welding but these measures only are still not satisfactory. Further, it is also important to reduce the N content to the lowest degree. This is because the N in the Ni base alloy containing a large amount of Al easily forms Al nitride as described above, and these nitrides precipitation significantly inhibit hot workability and weldability.